The bold claim by North Korea on Wednesday that it had detonated a hydrogen bomb appeared to fizzle under the intense scrutiny of U.S. physicists and nuclear weapons experts.
The explosive power detected by earthquake sensors around the world was much weaker than would be expected from a hydrogen bomb, experts said.
"It was not very big," said Philip Coyle, the former director of nuclear weapons testing at the Nevada Test Site and a longtime U.S. national security official.
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An exhaustive investigation into the test, including sampling air for telltale radioactive particles and studying seismic shock waves, will take weeks. The analysis is likely to confirm the size of the detonation, the type of radioactive fuel it used, how the fuel was produced and the sophistication of its design.
U.S. experts initially estimated the power of the underground explosion was as small as 6 kilotons, less than the atomic bomb that the U.S. dropped on Hiroshima in World War II.
An analysis by the U.S. Geological Survey fixed the coordinates of the detonation in the northeast sector of North Korea, an isolated and heavily forested area about 6,000 feet in elevation. The nearest city or town is 14 miles away.
If North Korea has developed a hydrogen bomb, it would mark a military breakthrough by the communist nation that would allow it to eventually build highly destructive and lightweight warheads that could be delivered by missiles over thousands of miles. Any atomic bombs in its inventory today would probably be so heavy that only bombers could carry them, limiting their threat to the region.
Adm. William Gortney, head of North American Aerospace Defense Command and U.S. Northern Command, said last year that the Pentagon believes North Korea has the ability to produce so-called miniaturized nuclear warheads and can deliver them on ballistic missiles, though they would fall well short of the U.S. mainland.
The White House said the test had not changed its assessment of North Korea's technical and military capabilities.
The most basic atomic bomb is a type of gun that shoots pieces of highly enriched uranium at each other. A more sophisticated design uses a hollow sphere of uranium that is imploded with conventional explosives. Whether North Korea has even achieved the more sophisticated implosion-type weapon has never been disclosed by U.S. officials.
By contrast, a hydrogen bomb derives massive explosive force by fusing hydrogen atoms together, somewhat like the nuclear reaction that occurs in stars, and using radiation to create additional fission.
The progress of moving from simple atomic devices to full-scale hydrogen bombs takes enormous industrial and scientific resources that most experts say North Korea lacks.
The U.S. developed the first atomic bomb in 1945 and then took seven more years to conduct the first full-scale hydrogen bomb test in the Pacific, known as Ivy Mike, according to a book co-authored by Coyle. The detonation was so powerful at 20 megatons that it created a crater about 6,000 feet in diameter and vaporized an island after its inhabitants had been relocated.
Similarly, the move from atomic to hydrogen bombs took Russia six years, the United Kingdom five years, France eight years and China three years.
With less power and more weight, an atomic bomb is typically carried by bombers that would lack the range and penetration capability to threaten any nation but North Korea's immediate neighbors.
One interim step to a hydrogen bomb is known as a "boosted weapon," in which an implosion type of atomic bomb uses isotopes of hydrogen to derive some of its yield from fusion.
"In that case, it's something that they should be proud of and something that makes our security situation much worse," said Jeffrey Lewis, a nuclear expert at the Middlebury Institute of International Studies in Monterey.
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The North Korean blast registered on seismographs at around 5.1 in magnitude, about the same as North Korea's last test in 2013. But the tremors caused were twice that of North Korea's test in 2009, which translates to several kilotons of TNT.
Expert assessments of the explosive yield range from 6 to up to 10 kilotons for the 2013 test. If North Korea had actually tested a hydrogen bomb, the yield of the device probably would have been tens of kilotons more powerful than what current measurements are indicating, said Kingston Reif, a nonproliferation expert with the Arms Control Assn. in Washington.
"One indication of whether the bomb might have been a hydrogen bomb is the size of the explosion," he said. "Determining what kind of fissile material and other isotopes were used for the bomb is another indicator."
The Comprehensive Nuclear-Test-Ban Treaty Organization, a multinational organization which monitors for nuclear testing worldwide, said that 27 of its land-based monitoring stations detected the event, but that analysts were still sifting through the data.
"We need a couple of days to be able to come back with what we call the smoking gun, that we can correlate from our member states on the conclusion of the event and this is what we have been working," Lassina Zerbo, head of the Vienna-based organization, said Wednesday at a news conference.
In 2013, the organization's monitoring stations did not detect radioactive particles until mid-April, after a test that took place in February.
The Air Force Technical Applications Center at Patrick Air Force Base in Florida was analyzing captured data on the seismology and was ready to launch WC-135 Constant Phoenix, the so-called nuclear sniffer plane, to detect any radioactivity.
"It was almost instantaneous that our seismic equipment detected the event," said Susan Romano, an Air Force spokeswoman. "But finding out exactly what happened will take time."
Vartabedian reported from Los Angeles and Hennigan from Washington.
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